Image: Fiorillo, A. R.; Tykoski, R. S.
Meet Nanuqsaurus hoglundi, the newest member of the tyrannosaur family. Discovered in northern Alaska, the pygmy carnivore is 70-million-years young and measures about half the size of its famous cousin, the Tyrannosaurus rex. It was classified as a new species this week in the journal PloS ONE, by researchers based out of the Perot Museum of Nature and Science in Dallas.
“The new tyrannosaurid fills both evolutionary and paleographic gaps in our knowledge of the highly derived tyrannosaurine theropods,” the team said in the paper. “The new form inhabited the northern-most margin of Laramidia in the early Late Cretaceous, and serves to expand our understanding of tyrannosaurid adaptability and diversification.”
In other words, the diminutive tyrannosaur was an Arctic dweller, and its classification broadens our understanding of what life was like for animals pushed to the colder edges of Mesozoic Earth. When we think of the Cretaceous, tropical jungles and swampy meadows are often the first environments that jump to mind—and indeed, that's where most of the “superstar” species thrived.
But the dinosaurs that found a niche around the polar regions are a fascinating bunch as well, and they can provide valuable context in analyzing links between climate change and body size. This has become a more heated (ha!) subject recently, since scientists have discovered a conclusive relationship between many other ancient species and climate change.
As the planet's thermostat continues to rise, paleontologists have a lot more incentive to study the hell out of two other major warming events in prehistory: the Paleocene-Eocene Thermal Maximum, which occurred about 56 million years ago and lasted for about 170,000 years, and its successor the Eocene Thermal Maximum 2, which occurred 53 million years about and endured for about 100,000 years.
There is strong evidence suggesting that mammals went through a period of evolutionary dwarfism during both of these events. And it wasn't just milk-makers that skewed small in hot weather—in 2009, geologists based in Kansas discovered that ancient soil biota also decreased in size by 30-46% during the PETM.
This tendency for a wide variety species to reduce their body size during warming periods makes sense if you look at extant tropical fauna. Smaller bodies don't retain as much heat as larger ones, which is why you see gigantism at the poles and dwarfism along the equators. That's called Bergmann's Rule, and—getting back to our tiny tyrannosaur—it appears that dinosaurs might rebelliously break it.
The Cretaceous Arctic was much warmer than it is today, and had a climate somewhat like Seattle. Even so, dark, freezing winters would have been very difficult for marauding tyrannosaurs, which might explain why Nanuqsaurus was unable to feed a frame as large as his equatorial relatives. But as paleontologist Matt Lamanna noted in NatGeo, it's still weird that dinosaurs seem to have the exact opposite pattern to birds and mammals.
"It's a pretty surprising discovery," said Lamanna. "In the modern world you tend to find big stuff at the poles, and it's interesting that in the Cretaceous world that may not hold true."
Along those lines, Nanuqsaurus is far from the only dinosaur that survived cold weather by staying small. The Troodon, a frontrunner for the smartest dinosaur that ever lived, shared the Cretaceous Arctic with the small tyrannosaur. Though it had a large brain, Troodons weighed only about 110 pounds and grew to about three feet tall.
Perhaps dinosaurs will be the exception that proves Bergmann's Rule, or perhaps the relationship between climate change and body size is too complex for a one-size-fits-all solution. For now, let's focus on the one thing we can all agree on: Nanuqsaurus is freaking adorable.